The present invention relates to electromagnetic energy heating systems, and more particularly, to microwave systems which heat in a single operation a plurality of items not all of which are to be heated to the same temperature.
In institutions, such as hospitals, it is desirable to provide a means whereby a complete meal may be brought up to serving temperature in a single, quick operation. If this can be done, meals may be prepared in an assembly-line fashion on weekdays and during normal working hours. The meals may then be kept refrigerated until the time when they are to be served.
The temperature at which food items are to be served varies according to custom and taste but generally water for a hot beverage should be near its boiling point. A thick soup may be served at about 160.degree. F. whereas a thin soup or broth is perferably somewhat warmer, for example, at about 170.degree. F. The temperature of an entree is controlled, to some extent, by its type since certain meats and vegetables do not tolerate heat as well as others. Potatoes and beef, for example, should be in a range of 140.degree. F. to 160.degree. F., but chicken is satisfactory as warm as 190.degree. F. A starch dish, such as potatoes, is preferably heated to about 170.degree. F.
In recent years, microwave ovens and other electromagnetic heating devices have come into widespread use for the quick cooking or reheating of food items. These ovens typically include a rectangular oven chamber having a door through which food items may be placed into the chamber for cooking or heating. Such ovens normally include a timer which may be set to control the length of the time during which electromagnetic energy is supplied to food items placed within the oven chamber. Such an oven is not normally suitable for heating a complete meal in a single operation because all of the food items present within such an oven are heated to about the same temperature.
Many attempts have been made to devise means whereby the amount of energy supplied to the various food items within the oven chamber may be controlled so that an entire meal can be brought to serving temperature in a single heating operation. The prior art is replete with various systems for achieving this end which involve either the shielding of food items from electromagnetic radiation or the incorporation into the food of some energy-absorbing item which subjects particular food items to additional heating. For example, Peter N. Stevenson discloses in his U.S. Pat. Nos. 3,547,661 and 3,615,713 the technique of enclosing food items which are to be completely shielded from radiation in a conductive wrapper or the like and wrapping items which are to receive less heat in conductive wrappers containing perforations which allow greater or lesser amounts of energy to reach the various food items in accordance with their nature. This technique for controlling the heating of the various items is relatively expensive to implement, since it requires the various food items to be individually wrapped in a rather specialized manner or, alternatively, requires the provision of specially constructed metallic shields for the food trays which are to be heated. As another example, U.S. Pat. No. 3,353,968 discloses a method of using dielectric material to supply more heating energy to certain items than to others. U.S. Pat. No. 3,302,632 discloses a food tray having separate compartments for different food items under which compartments are buried metallic grids which reduce the amount of energy reaching the food items in the corresponding compartments. Numerous other such arrangements are known, but in general they all require the use of either a very special form of tray having shielding or energy concentrating elements embedded therein or else the selective wrapping and packaging of the food items which are to be placed upon the tray. All of these arrangements increase the cost of prepackaging meals and, consequently, are undesirable.